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A Study on Equilibrium and Kinetics of Carbocation-To-Carbinol Conversion

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A Study on Equilibrium and Kinetics of Carbocation-To-Carbinol Conversion Indian Journalof Chemistry Vol.39A, July 2000, pp.703-708 A study on equilibrium and kinetics of carbocation-to-carbinol conversion for di- and tri- arylmethane dye cations in aqueous solutions: Relative stabilities of dye carbocations and mechanism of dye carbinol formation Susanta K Sen Gupta', Sangeeta Mishra & V Radha Rani Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India Received 17 August 1999; revised 7 April 2000 Arylmethane dye cations form a structurally interesting set of stable carbocations. A detailed study on rate-equilib­ ria of carbinol formation from two diarylmethane and nine triarylmethane dye carbocations in aqueous solutions has been carried out using spectrophotometric measurements. The conclusions reached are : (i) The stability order found (auramine O>crystal violet = methyl violet > victoria blue R > victoria pure blue BO = ethyl violet > pararosaniline > brilliant green > malachite green > carbocation form of Michler's hydrol > methyl green), seems to be determined by an interplay of dye carbocation / carbinol conformation and stereoelectronic effects of substituents; and (ii) carbinol formation is general base catalysed and occurs by the rate determining attack of a HzD molecule on the dye carbocation centre via two kinetic pathways one mediated by another H 0 molecule and the other by a OR ion. 2 Rate-equilibrium parameters and structure-reactivity libriJ.lmof arylmethane dye carbocation-to-carbinol con­ correlations for carbocation-to-carbinol conversion pro­ version in aqueous media was reported excepting the vide a valuable basis for the analysis of mechanisms of instances of crystal violet and malachite greenH•9 and of reactions involving carbocation intermediates. One some preliminary studies on carbinol formation from structurally interesting class of carbocations is provided several such dyes10.13; whereas carbinol formation from by 4-amino or alkylamino arylmethane dye cations. various other carbocations has been the subject of many 2 Highly electron donating 4-aminoaryl groups endow a systematic investigations�·9.11.1 .19. To wards bridging this methyl cation remarkable stability in aqueous solutions. gap a detailed investigation on rate-equilibrium of dye Further, steric crowding by aryl groups around the cen­ carbocation-to-carbinol conversion in aqueous solutions tral carbon makes reactions of these methyl cations with was undertaken with the objectives of determining sta­ a nucleophile considerably slower than those of most of bility scales, structure-reactivity relationships and other cations of similar stability. In fact, the rate of con­ carbinol-formation mechanisms for a set of eleven di-/ version of 4-amino or alkylamino di- or tri-arylmethane tri- arylmethane dye·cations. dye carbocation to the carbinol is sufficiently slow for monitoring the kinetics by conventional spectrophoto­ , The selected dye cations are : Carbocation form + metric techniques. Moreover, a significant number of of Michler's hydrol (MH; 4,4'-bis NMe2 Ph2C H), + structural effects due to carbocation structure, auramine 0 (AR; 4,4'-bis NMe2 Ph2C NH2), pararosa- " + stereoelectronic influences of aryl ring substituents, hy­ . nHine (PR; 4,4',4 -tris NH2 Ph]C ), malachite green + ' dration of the substituent and the carbocation centre etc. (MG; 4,4'-bis NMe2 Ph3C ), brilliant green (BG; 4,4'­ + are encountered in the reaction. Arylmethane dy e cat­ bis NEt2 Ph3C ), methyl violet (MV; 4,4'-bis NMe2 - " + " ions are well-known reagents for solvent extracti<:>n-spec­ 4 -NHMe Ph3C ), crystal violet (CV; 4,4',4 -tris NMe2 + " + trophotometric determination of a number of metals and Ph,C" ), methyl green (MeG; 4,4'-bis NMe2 -4 -N Me2 Et 2 Ph C+), " + non-metals 1. Their carbinol forms are sufficiently ba­ 3 ethyl violet (EV; 4,4',4 -tris NEt2 Ph C ), " 3 sic and potential reference bases for studying proton victoria blue R (VBR; 4,4'-bis NMe2 Ph2-4 -NHEt 1- + transfer from various acids in apolar aprotic solvents3.? NaphC ), and victoria pure blue BO (VBP; 4,4'-bis NEt2 " + Despite such importance no detailed study on rate-equi- Ph2 -4 -NHEt I-NaphC ). 704 INDIAN J CHEM. SEC. A, JULY 2000 Materials and Methods BG, 5.46 fo r MV, 5.19 for CV, 6.13 for MeG, 6.03 for The di- and tri- arylmethane dye samples employed EV, 5.27 for VBR and 2.70 for VBP at their respective were from Aldrich, BDH, E. Merck or Sigma. Ten of A the eleven samples were obtained in their cationic (R+) max. form viz. auramine 0, pararosaniline, malachite green, Results and Discussion brilliant green, methyl violet, crystal violet, methyl green, Dye carbocation - carbinol equilibration ethyl violet, victoria blue R and victoria pure blue BO. However, the remaining sample was obtained in its The equilibrium constant (KR +) for dye . + . colourless carbinol (ROH) from Michler's hydro 1. It was carb ocatlOn (R )-to-carbmol (ROH) conversion in aque- completely converted to its coloured cationic from (MH) ous solutions: by dissolving in a HCI solution of pH -5 and adjusting ROH + H+ the pH for the maximum absorbance of the cationic form . ..(1) of Michler's hydrol (MH) at its , 604 nm. Am ax . ..(2) The buffers used for carbocation-carbinol conversion (the activity of water being defined as unity in dilute for the above dyes were phosphate (KHl04-NaOH) aqueous solutions) was determined as : buffers for pH, 5.50-9.00 and borate (Na B 4 0 -NaOH) buffers for pH, 9.20-11.60. pH was measured2 7 in a 335 ... (3) Systronics digital pH-meter. + where All and Ac are absorbances of the dye cation (R ) initially and at equilibrium at its The Debye-Hiickel Equilibrium and kinetics of carbocation-to-carbinol A.nax conversion for a dye cation (R+ ) were studied by mea­ correction for activity coefficients, -Alz+llzt'-'flat the ionic suring absorbances of the dye cation in buffer solutions strength (fl) of 0.0 1 M for the buffer media used was not of appropriate pH range each maintained at the ionic significant enough to be incorporated in Eq.3. Values of strength of O.OIM, at 35.0 ± O. loC for an initial dye cat­ pKR+ for the select dye carbocations are reported in ion (R+) concentration (-4 x 1O.5M) corresponding to Table I. the absorbance of Abs. at its measuring wave length As seen from the data in Table I, stability of the dyes I measured by their pKR varies considerably spanning a (A ) in a 160 A Shimadzu UV-vis recording spectro­ + photometerm with cuvet thermostatting facility. The ab­ range of 4.4 pKR+ units and depends significantly on sorbance of a dye cation (R+) falls with increase in pH the type and number of aryl rings contiguous to the due to increase in conversion to the colourless carbinol carbocation centre as well as on the nature and number of -donor substituents at the 4-position of the aryl rings. (ROH). The initial absorbance (Ao) for the calculation 1t of equilibrium and rate constants (Eqs 3 and 6) was then The order of stability among the dye carbocations in aqueous solutions is thus : the limiting maximum absorbances at its A on lower- ing the pH of the medium. The pHs chosen maforx the study + AR > CY MY > YBR > YBP EY > PR > BG> MG > MH > were not on the strongly acidic side so that R -ROH = = MeG ...(4) equilibration was freed from participation of species like RH2+, RHOH+. Absorbances were measured at 604 nm The effects of structure on dye carbocation stability for MH, 540 nm for PR, 615 nm for MG, 630 nm for Stability of a dye cation towards a nucleophilic at­ BG, 580 nm for MV, 590 nm for CV, 632 nm for MeG, tack should depend primarily upon the degree of direct 595 nm for EV, 610 nm for VBR and 615 nm for VBP. resonance interaction between its cabocation centre and However, for AR which undergoes conversion to both the lone pair of electrons on the nitrogen atom of 4-aminol carbinol and imine forms, measurements of absorbance alkylamino substituents on the aryl rings attached to it. were made simultaneously at three wavelengths : 315 A perusal of the dataonpK + and (sum of the Brown nm, 370 nm and 430 nm corresponding to imine form, R I,cr+ substituent constants 14.15 expressing exalted resonance carbinol form and cationic form of the dye AR respec­ properties for the 4-substituents over all the aryl rings) tively (discussed in Appendix).The molar absorption in Table 1 shows clearly that there is no overall general coefficients (104 dm3 mol·1 cm·l) for the dyes were : 9.53 correlation between pK + and e.g. although the set for MH, 2.86 for AR, 6.46 for PR, 6.24 for MG, 5.07 for R I,cr+ Table 1- Equilibrium and kinetic parameters for dye carbocation-to-carbinol conversion at 35�. 1 °C Dye carbocations + Log k Log k Chemical name 4-aryl ring aLO pH range pKR + OH H2 substituents b(±O.03) (M·lmin·l) (min·I) 0 (±O. I) (±O. I) b O b O Cation form Michler's hydrol (MH) 4,4'-bis dimethylamino diphenyl carbocation Two-NMe2 -3.40 5.72-7.20 5.92 7.34 -0.76 en Auramine 0 (AR) 4,4'-bis dimethylamino diphenylamino carbocation Two-NMe2 -3.40 9.80-11.00 9.88 0.13 -2. 10 tTl Z Pararosaniline (PR) 4,4',4"-tris amino triphenyl carbocation Three-NH2 -3.90 7.00-9.06 7.27 2.64 -1.70 Q C Malachite green (MG) 4,4'-bis dimethylamino triphenyl carbocation Two-NMe2 -3.40 6.50-8.02 6.55 2.66 -1.69 � Brilliant green (BG) 4,4'-bis diethylamino triphenyl carbocation Two-NEt2 -4.
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